1. Field Of The Invention
The present invention relates to the field of digital image storage and display systems.
2. Prior Art
In prior art digital image storage and display systems, the information for each pixel in the image to be displayed is stored sequentially in digital form, and for display purposes is repetitively read out in synchronism with a raster scan display device so as to repetitively redraw the desired image on the display screen. In the case of a simple black and white image wherein each pixel is to be displayed either as black or white, but no of grey shades therebetween, only a single bit of storage is required for each pixel location. Since the digital information is generally stored in the order in which the pixels are displayed, one can relate an "area" (address range) of memory space with the corresponding two dimensional image or image area being displayed. Because only a single bit is required to represent each pixel, the memory area is referred to as a memory plane, in essence corresponding to the planar area of the image itself.
If on the other hand, one also desired not only black or white, but two grey shades therebetween, a second memory plane may be added, covering the same address range as the first memory plane. Now each bit in any memory location in the first memory plane has associated therewith a corresponding bit at the same location in the second memory plane, so that the two will be addressed and processed simultaneously and used together to determine white, black or appropriate grey shade. In general, one might use, for instance, eight bits per pixel for a relatively fine control of grey shade, in which case there would be eight memory planes, all simultaneously addressed to bring up the information for display. (While the information is displayed pixel by pixel, an eight bit wide memory organization will bring up information for eight bits at a time on each fetch, a sixteen bit wide memory for sixteen pixels at a time, etc.) In general, these memory planes can be equated to image planes in the sense that generally in an eight bit grey scale value for each pixel, the most significant bit will have the most contribution to the grey shade, the next significant bit providing half the contribution of the most significant bit, etc., with the least significant bit providing only a fine adjustment of the general grey shade determined by all of the more significant bits. In this sense, each memory plane defines a two dimensional image contribution which in theory may be displayed alone and which when displayed with the contribution of the other memory planes will provide the grey shade image as desired.
In the case of color images, there are a number of data formats. One such format, referred to as RGB, is based on mixing three primary colors, each controlled in intensity to provide the colors and shades desired. In such a format, certain bits are assigned for control of red, certain bits for the control of green and certain bits for the control of blue, each color being controlled substantially the same as one would control a corresponding grey shade image. P Also in the prior art, additional memory planes are frequently provided for other purposes. By way of example, one or more planes may be provided to provide a grid or other overlay over an image. For instance, in a color system, two bits of overlay might be used, so that for each pixel location on the image, either the respective image pixel will be displayed or an overlay pixel will be displayed dependent upon the two bits of overlay information stored at the corresponding memory location in the overlay memory planes. Typically, any non-zero two bit code for any pixel location will cause an override of the pixel data by the overlay data, with the three variations of non-zero overlay data for the pixel location determining the color of the overlay pixel, whereas a zero in each overlay plane for that pixel location would not affect the direct display of the image pixel data. Similarly, additional memory planes may also be provided which define a clip mask, an underlay, the color display mode (RGB or other format) for that pixel, etc. These memory planes too are simultaneously addressed with the corresponding image planes, so that all data defining a pixel, or a plurality of pixels dependent upon the width of the display memory, are simultaneously available and clocked out pixel by pixel to define each pixel in the image.
As shall subsequently be seen, the present invention departs from this typical organization, and can provide various advantages with respect to memory utilization, memory bandwidth requirements and speed of changing such things as an overlay.